The present invention relates to a method for treating calcium borate ores to obtain useful boron-containing compounds. The invention also relates to the use of the resulting boron-containing compounds in the treatment of wood and other related wood-based products to protect such products against fungal attack, and against attack by subterranean termites and wood-boring insects such as powder-post beetles and carpenter ants. The invention further relates to the use of the boron-containing compounds to improve the flame retardancy of wood and wood products. Still further, the invention relates to the boron-containing compounds prepared by the inventive method for treating calcium borates.
Typically, wood and other cellulose products such as plywood and oriented strand board (OSB), are exposed to a wide range of weather and environmental conditions. During the useful lifetimes of such products, they may also become exposed to fungi, and/or they may become prey to various boring or wood-eating insects. Similarly, such products may become exposed to fire. Such exposure to fungi and insects hastens the degradation of the products. Exposure to fire or flames increases the risk of loss of life and property.
Methods and compositions for treating wood and cellulose products to provide at least some protection against such conditions are known in the art. In one such method, cellulose products are treated with a composition comprising sodium sulfate and ammonium pentaborate. This composition is generally obtained as a result of the reaction between sodium borates (borax) and ammonium sulfate in water. However, ammonium pentaborate is soluble in water. Therefore, the composition gradually leaches out of the treated wood or cellulose upon repeated exposure to outdoor moisture, such as rain.
Attempts have been made to minimize the leaching of the ammonium pentaborate by adding soluble calcium salts to the sodium sulfate/ammonium pentaborate composition, to obtain a second set of reaction products that is less prone to leaching out of the wood or wood products. However, results with the calcium salts have met with only limited success, as the post-addition of the calcium salts to the sodium sulfate/ammonium pentaborate composition generates insoluble calcium compounds and/or calcium borates, causing those insoluble products to precipitate out of the solution before application. Besides making the application more difficult, the above-described process can remove both calcium and borates from the composition to be applied to the wood, thereby decreasing the effectiveness of the composition. In order to avoid this condition a second set of reaction conditions would be required (secondary application to treated wood), thereby necessitating an additional treatment step to produce the desired products.
Calcium borate ores have previously been used as components in dry powder flame-retardant formulations. One such use was described in U.S. Pat. No. 3,865,760, to Pitts, et al., wherein the ore colemanite (or alternatively, the ores ulexite or pandermite) was used as a filler in a rubber and plastic dry powder formulation, alone or in combination with alumina trihydrate and calcium carbonate. In this formulation, high levels of unreacted dry colemanite were required in order to receive the desired flame-retardant effect.
Another such use was described in U.S. Pat. No. 4,076,580 to Panusch, et al. This patent discloses a process for producing flame-retardant cellulosic board, comprising treating the board with a xe2x80x9csynergistically actingxe2x80x9d composition consisting of alumina hydrate and ulexite. The combination requires loadings for flame retardation at high levels that sometimes interfered with the board properties.
Another use was described in U.S. Pat. No. 4,126,473 to Sobolev, et al. This patent discloses a three-component flame-retarding agent consisting of an xe2x80x9caluminousxe2x80x9d material, a boron-containing mineral such as colemanite or ulexite, and a xe2x80x9cco-synergistxe2x80x9d, namely a phosphate or sulfate-containing inorganic salt.
The use of the calcium borate ores colemanite and ulexite as a termite bait was described in U.S. Pat. No. 4,363,798 to D""Orazio. This patent describes a method for protecting a structure from termites, in which a composition comprising wood inoculated with brown rot fungus is used as an attractant. The wood is ground into sawdust, and mixed with a boron-containing toxicant, such as colemanite or ulexite. The termite bait or attractant is then placed in close proximity to a wood structure to be protected, so that termites will be attracted to the bait.
Attempts have also been made to obtain treatment solutions for either flame retardation or for the protection from wood decay fungi, termites and wood-boring insects using water-soluble sodium borates. However, since sodium borates are highly soluble in water, these products do not provide adequate resistance to leaching after the application to wood or wood products.
U.S. Pat. No. 4,873,084 to Sallay discloses an insecticidal composition utilizing ammonium pentaborate and a mildewcide. This patent discusses the insecticidal activity of ammonium pentaborates. Ammonium pentaborates are precursors to boric acid, which is formed during ingestion by the insect. The patent states that certain calcium and barium triborates act in a similar fashion to produce boric acid in vivo in insects, the boric acid being toxic to insects. In the process described in this patent, wood previously treated with ammonium pentaborate is secondarily treated with various barium and calcium salts to cause a chemical change to a less soluble barium and/or calcium borate product. A mildewcide such as Busan(copyright) or Tyrosan(copyright) is added to control wool-boring insects and wood decay fungi.
Another Sallay patent, namely U.S. Pat. No. 4,514,326, discloses a flame retardant composition comprising ammonium pentaborate, and an alkali and/or alkaline earth metal sulfate, sulfite, hydrophosphate, or mixtures of them. The reaction products are produced by heating an aqueous suspension of a metal tetraborate ore, and then reacting the resulting product with an ammonium salt, such as ammonium sulfate. The disclosure states that the role of the by-product alkali salts is to increase the solubility of ammonium pentaborate in water. This patent does not address the problem of leaching away of soluble ammonium pentaborate that occurs with the use of water-soluble borates.
According to the present invention, leach-resistant compounds are produced from a reaction between naturally occurring calcium borate ores, such as colemanite, and an acid, such as acetic acid; and then treating the reaction products with ammonia. When applied to substrates such as wood and other cellulose products, the resultant compounds in a water-based solution provide enhanced flame retardancy protection, and provide enhanced protection against attack by wood decay fungi and insects.
The present invention utilizes naturally-occurring calcium borate ores that are extremely insoluble in water, and produces a compound or compounds that are soluble in water, thereby facilitating the preparation of the treatment solution, and the resulting application of the solution to the wood. The treated wood shows dramatic resistance to leaching by water after treatment and drying.
The invention advantageously utilizes the naturally bound calcium of the calcium borate ores to provide the increased resistance to leaching without the need of any post-treatment after the initial application. The calcium compound or compounds that are produced are placed in solution, and thereafter remain in solution. The compound or compounds so produced are apparently just as stable in solution as other borates similarly made from water-soluble sodium borates. Upon cold precipitation, the compounds may be reintroduced into solution upon heating.
It has been discovered that when certain naturally-occurring calcium borate ores, such as colemanite, are reacted with an appropriate acid, such as acetic acid, and the resulting reaction products are further reacted with ammonia, that the calcium from the ores is placed in solution.
When the solution containing the dissolved calcium is thereafter applied a substrate, such as wood or other cellulose-based products such as plywood, particle board or oriented strand board (OSB), the resulting compounds of the solution have superior leach resistance when compared to the leach resistance obtained by other water-based compounds, such as water-based sodium salts. When utilizing the dissolved-calcium solution of the present invention in this manner, a high level of leach-resistant boron remains in the substrate even after prolonged exposure to the elements. As a result, the treated wood is protected against attack from wood decay fungi, and from subterranean termites and wood-boring insects such as powder-post beetles and carpenter ants. Furthermore, the retention level of boron in the substrate surpasses the level required for visible exposure of the treated wood. In addition, treating such substrates with the inventive solution provides enhanced flame retardancy protection.
According to the present invention, calcium borate ores are reacted with a specified acid to produce the calcium acid salt and boric acid in solution. These reaction products are then treated with a slight excess of anhydrous ammonia or ammonium hydroxide to produce ammonium pentaborate and the calcium acid salt.
The mechanism of this reaction in water is illustrated by the following two-step process utilizing colemanite, empirical formula Ca2B6O11.5 H2O, as the borate ore, and acetic acid as the acid component:
1) Ca2B6O11.5 H2O+4 CH3COOHxe2x86x922Ca(OOCCH3)2+6 H3BO3 
2) 2 Ca(OOCCH3)2+6 H3BO3+1.5 NH4OHxe2x86x921.2(NH4)B5O8.4 H2O+2 Ca(OOCCH3)2+0.3 NH4OH
For optimal results, the reaction between the borate ore and the acid should be substantially completed prior to the addition of the ammonia.
The preferred ore for use in the present invention is colemanite. Although colemanite is a borate ore, it is chemically considered as a carbonate. When colemanite is reacted with glacial acetic acid or a strong mineral acid, the typical carbonate reaction of profuse foaming is produced. This foaming effectively cools the reaction, and makes contact between the acid and the colemanite much more difficult. In its natural states, colemanite ore includes calcite. When contacted with an acid, calcite produces the typical acid/carbonate reaction. This reaction produces carbon dioxide gas, which in turn makes the foam:
Acid+CaCO3xe2x86x92CO2↑+H2O+Ca+2+acid salt 
Although colemanite is the preferred ore, other borate ores that naturally contain calcium may also be used in the inventive process. A non-limiting list of such calcium borate ores includes ulexite, pandermite, danburite and datolite.
Acetic acid is the preferred acid for use in the reaction. However, other closely related organic acids, such as formic, oxalic and malonic acids, may also be used. Of the listed acids, acetic acid is the most preferred, followed in succession by formic, oxalic, and malonic acids. The above-mentioned acids are favored due to their dissociation constants Ka. The term pKa is defined as the xe2x88x92log Ka. The lower the value for pKa, the xe2x80x9cstrongerxe2x80x9d the acid, and the greater the degree of dissociation. Formic, oxalic and malonic acids all have pKa values less than that of acetic acid, and therefore, are stronger acids than acetic acid.
To facilitate the reaction between the ore and the acid, it is preferred to granulate the ore to a particle size of about 200 mesh or less. The granulated ore is then directly reacted with undiluted glacial acetic acid. Although diluted acids may be utilized, chemical literature suggests that the activity, or the level of dissociation of acetic acid, substantially decreases with dilution of the acid, thereby reducing the apparent reactivity. Thus, the use of diluted acids typically lengthens the reaction time.
If certain strong mineral acids, such as hydrochloric acid and nitric acid, were used to treat the calcium borate ores, and the reaction products were thereafter treated with ammonia, a solution containing highly-soluble calcium acid salts (e.g., calcium chloride and calcium nitrate) would be obtained. Due to the extreme solubility of these dissolved calcium salts, the leach resistance obtained when treating the substrate with these highly soluble salts would not be appreciably increased when compared to the leach resistance obtained using the water-soluble sodium borates.
In addition, treatment of the calcium borate ores with other strong mineral acids, such as sulfuric acid and phosphoric acids, and subsequent treatment with ammonia results in the formation of precipitated calcium acid salts of those acids (calcium sulfate and/or calcium phosphates). In this event, all, or nearly all, of the calcium is removed from the solution, thereby resulting in a product with little resistance to leaching.
A key aspect of the present invention is that the matrix of the inventive composition must retain a significant calcium content. Although it is the boron compounds that provide the actual protection to the substrate against fire, wood decay fungi and insects, these boron compounds must be provided in a matrix that substantially reduces their ability to leach out under commonly-encountered environmental conditions, such as high humidity, rain, fog, and exposure to soil moisture and dew. Calcium is believed to act as a fixative agent in this matrix to dramatically increase the resistance of the substrate to the leaching out of the boron compounds. Calcium salts obtained from the reaction between the calcium borate ore and acetic acid, and the subsequent treatment with ammonia, act to increase the resistance of the boron compounds to leaching out of the substrate. On the other hand, calcium salts obtained from the strong mineral acids listed above are either easily leached out of the substrate due to their high solubility, or they form precipitates prior to their application to the substrate. As a result, the level of protection against leaching provided by these salts of strong mineral acids is often inadequate.
When acetic acid is used in the reaction with colemanite, the calcium acetate salt is obtained. Calcium acetate salts are particularly preferred as they are less soluble than the extremely soluble salts obtained with hydrochloric and nitric acids, and yet are considerably more soluble than the substantially insoluble salts obtained with sulfuric and phosphoric acids.
Preferably, only a slight excess of residual ammonia is present in the formulation, as a slight excess of ammonia aids in keeping both the ammonium pentaborate and calcium salt in solution. It is preferred to avoid too great an excess of ammonia, because a large excess of ammonia will normally result in the formation of an objectionably large amount of insoluble calcium compounds, such as calcium hydroxide, lime or calcium tetraborate. Although the formation of a small amount of insoluble calcium compounds is permissible, the formation of these insoluble compounds has the undesirable effect of removing some of the calcium from the reaction solution. Accordingly, the pH of the reaction solution should preferably be maintained between about 5 and 9, more preferably between about 5 and 8, and most preferably between about 5.5 and 6.6. Preferably, water comprises the solvent for the reaction.
When preparing large batches of the inventive composition, it is preferred to mix previously combined portions of the initial ingredients. Sequentially adding xe2x80x9ccombined portionsxe2x80x9d of starting materials to a reaction vessel is a known practice in industry to facilitate the preparation of larger batches of combined reactants than might otherwise be obtained if large amounts of the initial ingredients were reacted directly with each other. For example, if one were to add large quantities of acetic acid, e.g. 1200 ml, directly to large quantities of colemanite, e.g. 3 pounds (1362 gm), in a reaction vessel the resulting mixture would be difficult to mix, and would foam profusely. However, when smaller xe2x80x9ccombined portionsxe2x80x9d are prepared, each containing previously-combined lesser quantities of acetic acid, e.g. 200 ml, and colemanite, e.g. 0.5 pound (227 gm), and added incrementally to the reaction vessel, the difficulties obtained when using larger portions of the respective starting materials are minimized.
In addition to the foregoing benefits, the use of combined portions of the ingredients also minimizes any adverse effects on the reaction that may be caused by other inorganic metals that are naturally present in the ore, such as magnesium, aluminum, iron and celestite (SrCO3), calcite (CaCO3), and silica. These inorganic metals may react with acetic acid to form water-soluble acetates, thereby reducing the amount of acid available to react with the colemanite portion of the ore. Reacting previously combined mixtures of colemanite and acetic acid is more effective, and improves the efficiency of the reaction.
Although not required, the addition of an oxidant, such as hydrogen peroxide, to the colemanite/acetic acid reaction solution may assist the acetic acid by artificially increasing its ionization, or xe2x80x9cstrengthxe2x80x9d. When large quantities of colemanite are to be reacted, the hydrogen peroxide may be introduced along with the combined portions of colemanite and acetic acid.
Cellulosic materials are hydrophobic, and therefore, are not easily penetrated by water and water-based solutions containing solids. Thus, it is also preferred to add a surfactant to the formulation. The use of a surfactant reduces the surface tension of water-based solutions containing solids, thereby aiding the penetration of the reaction products into the substrate being treated. Preferably, an amphoteric surfactant that can perform under either acidic or basic conditions is used. Amphoteric surfactants are particularly effective at the mildly acidic pH of about 6.0-6.5, which is within the preferred range in the present invention. The amount of surfactant added to the reaction mixture is based on volume and solids content. At less volume of solution, less surfactant is required. When the solution has high solids content, more surfactant is generally required, since solutions having higher solids content have a greater surface tension. The surfactant lowers the surface tension, thereby permitting the solution and solids to disperse into the cellulose, which is naturally hydrophobic. When a surfactant is employed, it is added to the reaction solution at the end of the formulation process.
The resulting solution containing the dissolved boron salts has an apparent B.A.E. (boric acid equivalent), which is based upon the amount of dissolved boron in the solution. Thus, the B.A.E. of the solution is substantially controlled by two variables, namely the quantity of the calcium borate ore (the source of the boron) used in the reaction, and the final volume of the solution.
After the inventive composition has been prepared, it may be applied to the substrate utilizing application methods generally known in the art for applying protective treatments to substrates, such as immersion and pressure treatment.
Examples 1-7, below, illustrate methods for preparing the inventive composition. The methods described in these examples are illustrative only, and are not intended to limit the scope of the invention in any manner. In each example, the calcium borate ore used was colemanite. The colemanite ore used in Examples 1, 2 and 7 originated from Turkey, while the colemanite used in Examples 3-6 originated from the United States of America. Colemanite ore obtained from different Sources will normally differ somewhat in purity. Therefore, the actual amount of boron and calcium present in a particular amount of colemanite, e.g., 1 kg, obtained from one source will differ somewhat from the amount present in 1 kg obtained from another source. If desires, a colemanite ore sample can be assayed in order to determine the amount of boron and calcium present in the particular sample.